A systematic study of idiopathic thrombocytopenic purpura in childhood is underway to determine the incidence and prognostic significance of circulating antiplatelet antibodies in this disorder. Antiplatelet antibodies are detected by a "platelet injury" technique involving release of C14-labelled serotonin from washed normal platelets after incubation with test plasma. The presence or absence of antiplatelet antibodies will be correlated with rate and incidence of recovery, nature of predisposing illness, response to corticosteroid therapy, response to splenectomy and response to immunosuppressive therapy.